U.S. patent application number 11/012003 was filed with the patent office on 2005-10-27 for process and apparatus for achieving single exposure pattern transfer using maskless optical direct write lithography.
This patent application is currently assigned to LSI Logic Corporation. Invention is credited to Callan, Neal P., Croffie, Ebo H., Eib, Nicholas K..
Application Number | 20050237508 11/012003 |
Document ID | / |
Family ID | 35136052 |
Filed Date | 2005-10-27 |
United States Patent
Application |
20050237508 |
Kind Code |
A1 |
Eib, Nicholas K. ; et
al. |
October 27, 2005 |
Process and apparatus for achieving single exposure pattern
transfer using maskless optical direct write lithography
Abstract
The present invention provides methods and apparatus for
accomplishing a phase shift lithography process using a off axis
light to reduce the effect of zero order light to improve the
process window for maskless phase shift lithography systems and
methodologies. A lithography system is provided. The lithography
system provided uses off axis light beams projected onto a mirror
array configured to generate a phase shift optical image pattern.
This pattern is projected onto a photoimageable layer formed on the
target substrate to facilitate pattern transfer.
Inventors: |
Eib, Nicholas K.; (San Jose,
CA) ; Croffie, Ebo H.; (Portland, OR) ;
Callan, Neal P.; (Lake Oswego, OR) |
Correspondence
Address: |
LSI LOGIC CORPORATION
1621 BARBER LANE
MS: D-106
MILPITAS
CA
95035
US
|
Assignee: |
LSI Logic Corporation
|
Family ID: |
35136052 |
Appl. No.: |
11/012003 |
Filed: |
December 14, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60565921 |
Apr 27, 2004 |
|
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|
Current U.S.
Class: |
355/67 ;
355/53 |
Current CPC
Class: |
G03F 7/70125 20130101;
G03F 7/70283 20130101; G03F 7/70291 20130101 |
Class at
Publication: |
355/067 ;
355/053 |
International
Class: |
G03B 027/54 |
Claims
What is claimed is:
1. A method of forming a pattern on a substrate, the method
comprising: providing a target substrate having formed thereon a
photoimageable layer; providing a mirror array comprising a
plurality of movable mirrors that can be configured to generate a
phase shift optical image pattern; configuring the mirror array to
generate a phase shift optical image pattern having a background
pattern; a line pattern; and an assist feature pattern;
illuminating the mirror array with off-axis light that is directed
onto the mirror array from an angle other than normal to a plane of
the mirror array to generate said phase shift optical image
pattern; and exposing the target substrate to said phase shift
optical image pattern to transfer a desired exposure pattern onto
the photoimageable layer.
2. The method of claim 1 wherein configuring the mirror array to
generate a phase shift optical image pattern having said background
pattern; said line pattern; and said assist feature pattern further
comprises: configuring the mirror array to generate the background
pattern so that the background pattern comprises an background
pattern optical signal having a light intensity above an exposure
intensity required to convert the photoimageable layer; configuring
the mirror array to generate the line pattern so that the line
pattern comprises a line pattern optical signal having a light
intensity below the exposure intensity required to convert the
photoimageable layer; and configuring the mirror array to generate
the assist feature pattern so that the assist feature pattern
comprises an optical signal having a light intensity above the
exposure intensity required to convert the photoimageable layer and
below the light intensity of the background pattern optical
signal.
3. The method of claim 1 wherein configuring the mirror array to
generate a phase shift optical image pattern having said background
pattern; said line pattern; and said assist feature pattern further
comprises: configuring the mirror array to generate the background
pattern so that the background pattern comprises an background
pattern optical signal having a light intensity below an exposure
intensity required to convert the photoimageable layer; configuring
the mirror array to generate the line pattern so that the line
pattern comprises a line pattern optical signal having a light
intensity above the exposure intensity required to convert the
photoimageable layer; and configuring the mirror array to generate
the assist feature pattern so that the assist feature pattern
comprises an optical signal having a light intensity below the
exposure intensity required to convert the photoimageable layer and
above the light intensity of the background pattern optical
signal.
4. The method of claim 1 wherein illuminating the mirror array with
said off-axis light is performed so that the mirror array is
illuminated with a light signal comprised substantially of first
order light.
5. The method of claim 1 wherein illuminating the mirror array with
said off-axis light is performed so that the mirror array is
illuminated with a light signal comprised substantially of non-zero
order light.
6. The method of claim 1 wherein illuminating the mirror array with
said off-axis light is performed by illuminating the mirror array
with an annular light beam.
7. The method of claim 1 wherein illuminating the mirror array with
said off-axis light is performed by illuminating the mirror array
with a light beam passed through a single off-axis aperture.
8. The method of claim 1 wherein illuminating the mirror array with
said off-axis light is performed by illuminating the mirror array
with a light beam produced by one of a quadrapole aperture, a
quadrapole quasar aperture, and an octopole aperture.
9. The method of claim 1 wherein providing a mirror array
comprising a plurality of movable mirrors includes providing a
mirror array having a plurality of movable piston mirrors that can
operate to generate a phase shift optical image pattern.
10. The method of claim 1 wherein providing a mirror array
comprising a plurality of movable mirrors includes providing a
mirror array having a plurality of movable cantilevered mirrors
that can operate to generate a phase shift optical image
pattern.
11. The method of claim 1 wherein providing a mirror array
comprising a plurality of movable mirrors includes providing a
mirror array having a plurality of movable tilt mirrors having a
quarter wavelength optical plate formed on a portion of each tilt
mirror and wherein said mirror array can operate to generate a
phase shift optical image pattern.
12. The method of claim 1 wherein configuring the mirror array
includes: configuring the mirror array to generate the background
pattern by arranging the mirror array such that mirrors producing
the background pattern have substantially no phase difference
relative to adjacent mirrors; configuring the mirror array to
produce the line pattern by arranging a first group of mirrors to
produce a phase difference of about 180 degrees relative to an
adjacent second group of mirrors; and configuring the mirror array
to generate the assist feature pattern by configuring the mirror
array such that one group of mirrors is oriented to produce a phase
difference of in the range of about 70 degrees to about 90 degrees
relative to an another adjacent group of mirrors.
13. The method of claim 12 wherein configuring the mirror array to
produce the line pattern is achieved by arranging a checkerboard
pattern of mirrors wherein each mirror in the pattern is arranged
to produce a phase difference of about 180 degrees relative to an
adjacent mirror in the checkerboard pattern.
14. A maskless lithography system comprising: a mirror array
comprising a plurality of movable mirrors that can operate to
generate a phase shift optical image pattern; an illumination
source configured for directing light along an off axis optical
path onto the mirror array to generate said phase shift optical
image pattern for projecting onto a target substrate; and a control
element capable of configuring the plurality of mirrors in a
desired arrangement capable of generating the phase shift optical
image pattern.
15. The maskless lithography system of claim 14 wherein the control
element configures the plurality of mirrors to generate a phase
shift optical image pattern having exposure features that include
sub-resolution assist features.
16. The maskless lithography system of claim 14 wherein the mirror
array is configured to generate a phase shift optical image pattern
having: a pattern of mirrors configured to generate constructive
interference; a pattern of mirrors configured to generate
destructive interference; and a pattern of mirrors configured to
generate an optical pattern that corrects for line width drift.
17. The maskless lithography system of claim 14 wherein the
illumination source is configured to generate substantially
coherent light.
18. The maskless lithography system of claim 17 wherein the
illumination source comprises a laser.
19. The maskless lithography system of claim 18 wherein the laser
produces light having a wavelength of 193 nanometers.
20. The maskless lithography system of claim 18 wherein the laser
produces light having a wavelength selected from among of 157
nanometers, 193 nanometers, and 248 nanometers.
21. The maskless lithography system of claim 14 further including a
stage configured to move the target substrate to facilitate
exposure of different portions of the substrate to the phase shift
optical image pattern.
22. The maskless lithography system of claim 14 wherein the mirror
array comprises a plurality of movable piston mirrors that can
operate to generate a phase shift optical image pattern.
23. The maskless lithography system of claim 14 wherein the mirror
array comprises a plurality of movable cantilevered mirrors that
can operate to generate a phase shift optical image pattern.
24. The maskless lithography system of claim 14 wherein the mirror
array comprises a plurality of movable tilt mirrors having a
quarter wavelength optical plate formed on a portion of each tilt
mirror and wherein such array of mirrors can operate to generate a
phase shift optical image pattern.
25. The maskless lithography system of claim 14 wherein the
illumination source includes a source aperture configured to direct
the light along the off-axis optical path to generate said phase
shift optical image pattern.
26. The maskless lithography system of claim 14 wherein the light
directed along the off axis optical path comprises substantially
first order light.
27. The maskless lithography system of claim 14 wherein the light
directed along the off axis optical path is comprised substantially
of non-zero order light.
28. The maskless lithography system of claim 25 wherein the source
aperture includes a quadrapole aperture having four openings for
generating off-axis light that is directed along the off-axis
optical path onto the mirror array to generate said phase shift
optical image pattern.
29. The maskless lithography system of claim 25 wherein the source
aperture includes an octopole aperture having eight openings for
generating off-axis light that is directed along the off-axis
optical path onto the mirror array to generate said phase shift
optical image pattern.
30. The maskless lithography system of claim 25 wherein the source
aperture is configured to generate an annular light beam off-axis
light that is directed along the off-axis optical path onto the
mirror array to generate said phase shift optical image
pattern.
31. The maskless lithography system of claim 14 wherein the mirror
array is configured to generate a phase shift optical image pattern
in a positive photoresist, the pattern having: a pattern of dark
lines having exposure intensities such that the optical energy
provided by the dark lines is below an exposure threshold for a
photoimageable material formed on the target substrate; a
background pattern of light having an optical energy above the
exposure threshold for the photoimageable material formed on the
target substrate; and a pattern of gray lines having exposure
intensities that are greater than those of the pattern of dark
lines but are such that the optical energy provided by the gray
lines is above the exposure threshold for the photoimageable
material formed on the target substrate wherein said pattern of
gray lines are generated by assist features of the mirror
array.
32. The maskless lithography system of claim 31 wherein the pattern
of dark lines is formed by configuring the mirror array to generate
a phase difference in a resultant light pattern of about 180
degrees in the portions of the mirror array corresponding to the
pattern of dark lines; wherein the background pattern is formed by
configuring the mirror array to generate a phase difference in a
resultant light pattern of about 0 degrees in the portions of the
mirror array corresponding to the background pattern; and wherein
the pattern of gray lines is formed by configuring the mirror array
to generate a phase difference in a resultant light pattern of in
the range of about 40 degrees to about 90 degrees in the portions
of the mirror array corresponding to the pattern of gray lines.
33. The maskless lithography system of claim 32 wherein the pattern
of gray lines is formed by configuring the mirror array to generate
a phase difference in a resultant light pattern of about 90 degrees
in the portions of the mirror array corresponding to the pattern of
gray lines.
34. The maskless lithography system of claim 31 wherein the pattern
of dark lines is formed by configuring the mirror array such that a
first group of mirrors is oriented to produce a phase difference of
about 180 degrees relative to an adjacent second group of mirrors;
wherein the background pattern of light is formed by configuring
the mirror array such that mirrors producing the background pattern
have substantially no phase difference relative to adjacent
mirrors; and wherein the pattern of gray lines is formed by
configuring the mirror array such that one group of mirrors is
oriented to produce a phase difference of in the range of about 40
degrees to about 90 degrees relative to an another adjacent group
of mirrors.
35. The maskless lithography system of claim 34 wherein the pattern
of gray lines is formed by configuring the mirror array such that
said one group of mirrors is oriented to produce a phase difference
of about 90 degrees relative to said another adjacent group of
mirrors.
36. The maskless lithography system of claim 25 wherein the mirror
array is configured to generate a phase shift optical image pattern
having: a pattern of dark lines having exposure intensities such
that the optical energy provided by the dark lines is below an
exposure threshold for a photoimageable material formed on the
target substrate; a background pattern of light having an optical
energy above the exposure threshold for the photoimageable material
formed on the target substrate; and a pattern of gray lines having
exposure intensities that are greater than those of the pattern of
dark lines but are such that the optical energy provided by the
gray lines is above the exposure threshold for the photoimageable
material formed on the target substrate.
37. The maskless lithography system of claim 36 wherein the pattern
of dark lines is formed by configuring the mirror array to generate
a phase difference in a resultant light pattern of about 180
degrees in the portions of the mirror array corresponding to the
pattern of dark lines; wherein the background pattern is formed by
configuring the mirror array to generate a phase difference in a
resultant light pattern of about 0 degrees in the portions of the
mirror array corresponding to the background pattern; and wherein
the pattern of gray lines is formed by configuring the mirror array
to generate a phase difference in a resultant light pattern of in
the range of about 40 degrees to about 90 degrees in the portions
of the mirror array corresponding to the pattern of gray lines.
38. The maskless lithography system of claim 37 wherein the pattern
of gray lines is formed by configuring the mirror array to generate
a phase difference in a resultant light pattern of about 90 degrees
in the portions of the mirror array corresponding to the pattern of
gray lines.
39. The maskless lithography system of claim 25 wherein the pattern
of dark lines is formed by configuring the mirror array such that a
first group of mirrors is oriented to produce a phase difference of
about 180 degrees relative to an adjacent second group of mirrors;
wherein the background pattern of light is formed by configuring
the mirror array such that mirrors producing the background pattern
have substantially no phase difference relative to adjacent
mirrors; and wherein the pattern of gray lines is formed by
configuring the mirror array such that one group of mirrors is
oriented to produce a phase difference of in the range of about 40
degrees to about 90 degrees relative to an another adjacent group
of mirrors.
40. The maskless lithography system of claim 39 wherein the pattern
of gray lines is formed by configuring the mirror array such that
said one group of mirrors is oriented to produce a phase difference
of about 90 degrees relative to said another adjacent group of
mirrors.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority of U.S. Provisional Patent
Application No. 60/565,921, filed 27 Apr. 2004, which is
incorporated herein by reference in its entirety for all
purposes.
[0002] This application is also related to the U.S. Utility patent
application Ser. No. 10/993,603 (Attorney Docket No.
04-0028/LSI1P245), filed on 19 Nov. 2004, entitled: "Process And
Apparatus For Generating A Strong Phase Shift Optical Pattern For
Use In An Optical Direct Write Lithography Process" which
application is incorporated herein by reference in its entirety for
all purposes.
[0003] This application is also related to the U.S. Utility patent
application Ser. No. 10/988,087 (Attorney Docket No.
04-0328/LSI1P247), filed on 12 Nov. 2004, entitled: "Process And
Apparatus For Applying Apodization to Maskless Optical Direct Write
Lithography Process" which application is incorporated herein by
reference in its entirety for all purposes.
FIELD OF THE INVENTION
[0004] The present invention relates to methods and apparatus for
forming semiconductor devices using maskless optical direct write
systems and methods. More particularly, the present invention
relates to methods and apparatus for using off-axis light to form
optical phase shift patterns that are directed onto substrates to
form substrate patterns.
BACKGROUND
[0005] Designers and semiconductor device manufacturers constantly
strive to develop smaller devices from wafers, recognizing that
circuits with smaller features generally produce greater speeds and
increased packing density, therefore increased net die per wafer
(numbers of usable chips produced from a standard semiconductor
wafer). To meet these requirements, semiconductor manufacturers are
involved in a continuous process of building new fabrication lines
at each new "next generation" process node (gate length). As the
critical dimensions for these devices grow smaller, greater
difficulties will be experienced in patterning these features using
conventional photolithography.
[0006] Conventional photolithography methods used for pattern
generation involve exposing a light sensitive photoresist layer to
a light source. The light from the source is modulated using a
reticle, typically a chrome-on-quartz mask reticle. During
processing, reticle patterns are transferred to a photoresist layer
formed on a semiconductor substrate. Commonly, such pattern
transfer is achieved using visible or ultraviolet light. The
exposed photoresist pattern is then developed to form a pattern of
photoresist on the substrate. The developed regions are then washed
away and the remaining photoresist pattern used to provide an
etching mask for the substrate.
[0007] One newer approach to achieving the desired critical
dimensions has been to use attenuated phase shift masks and strong
phase shift masks. Such masks have many useful properties. However,
such masks suffer from a number of shortcomings. Phase shifting
masks are very difficult to produce; and unlike binary masks, are
not readily reconfigurable. Additionally, conventional phase
shifting masks commonly require two or more exposures per substrate
layer to obtain a desired pattern. This has the effect of lowering
throughput to perhaps 40% of that achievable with a single exposure
approach.
[0008] An example of such a new process technology is embodied, for
example, in optical direct write process techniques. One example of
such a technique is taught, by the above-referenced inventors, in
U.S. patent application Ser. No. 10/825,342, entitled: "Optimized
Mirror Design for Optical Direct Write", filed on Apr. 14, 2004
(Attorney Docket No. LSI1P239/03-180) and hereby incorporated by
reference for all purposes.
[0009] An optical direct write system makes use of a programmable
mirror array to generate photolithographically reproducible optical
patterns that are projected onto a photoimageable layer. For
example, an optical beam is directed onto the mirror array at an
angle normal to the mirror array to produce an optical pattern. The
optical pattern is then projected onto a substrate with a
photoimageable layer. The reflected light pattern (i.e., reflected
from the mirror array) exposes the photoimageable layer to transfer
a desired pattern onto the substrate. Advantageously, the mirror
array of the optical direct write system can be reconfigured by
merely implementing software instructions to reconfigure the
arrangement and orientation of the mirrors of the array.
[0010] In some implementations, mirror arrays are configured to
generate phase shift exposure patterns. Typically,
photolithographic optical settings (commonly including focus and
dose, but not limited to such) and phase shifting mirror arrays are
optimized for a process to produce the best process window for a
given critical dimension. Commonly, a user/lithographer will
optimize the process window for the smallest critical dimension to
be found on the target substrate. Typically, this smallest critical
dimension is associated with smallest feature desired or is
associated with the smallest line pitch desired for a given process
layer. The settings are optimized to generate a process window
capable of faithfully reproducing the smallest feature with a
desired degree of fidelity. When the settings are optimized in this
way, they are generally excellent for reproducing dense line
pitches or very small features. However, due to the nature of phase
shift lithography, such optimized settings lose fidelity and
sharpness when applied to other critical dimensions or
significantly different line pitches. Thus, settings used to
produce dense line pitches and small critical dimension (CD)
features can be unsuitable for larger features. This is problematic
because a typical semiconductor has a healthy mix of feature sizes
and pitches. Thus, systems optimized for the worst case scenarios
(small CD's and dense pitch patterns) are not optimized for larger
features. This means that when systems optimized for dense patterns
or short line pitches are used for less dense patterns unintended
light scattering effects degrade the contrast and quality of the
image pattern. For example, by creating periodic ghost patterns of
alternating dark and light regions and causing drift in the width
and position of features. Such systems must be re-optimized to
image the larger features. This takes time and additional exposures
and accordingly reduces throughput for the affected systems.
[0011] In view of the above difficulties, what is needed is a
relatively simple and effective solution to such processing
difficulties.
SUMMARY OF THE INVENTION
[0012] To achieve the foregoing, the present invention provides a
lithography system configured to generate phase shift optical
exposure patterns which are directed onto a substrate. System
embodiments include a light source capable of generating off axis
light beams to improve the process window for image patterns
projected onto the target substrate to facilitate an optical
lithography process.
[0013] One embodiment of the invention involves a method of forming
a pattern on a substrate. The method involves providing a mirror
array comprising a plurality of movable mirrors and configuring the
mirror array to generate a phase shift optical image pattern having
a background pattern; a line pattern; and an assist feature
pattern. A target substrate having formed thereon a photoimageable
layer is provided. The mirror array is illuminated with off-axis
light to generate a phase shift optical image pattern. The target
substrate is exposed to the phase shift optical image pattern to
transfer a desired exposure pattern onto the photoimageable
layer.
[0014] In another embodiment, the invention comprises a maskless
lithography system. The system includes a mirror array having a
plurality of movable mirrors configurable to generate phase shift
optical image patterns. The system includes an illumination source
for directing off axis light beams onto the mirror array to form
phase shift optical image patterns that are projecting onto a
target substrate. The system includes a control element capable of
configuring the mirrors to generate phase shift optical image
patterns so that a phase shift optical image pattern thereby
created has exposure features that include sub-resolution assist
features.
[0015] These and other features and advantages of the present
invention are described below with reference to the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The following detailed description will be more readily
understood in conjunction with the accompanying drawings, in
which:
[0017] FIG. 1(a) is a simplified schematic diagram illustrating an
embodiment of a maskless optical direct write lithography system
constructed in accordance with the principles of the invention.
[0018] FIG. 1(b) is a more simplified schematic diagram
illustrating a portion of an embodiment of a maskless optical
direct write lithography system constructed in accordance with the
principles of the invention.
[0019] FIGS. 1(c)-1(e) are schematic diagrams illustrating aperture
embodiments suitable for use with a maskless optical direct write
lithography system constructed in accordance with the principles of
the invention.
[0020] FIG. 2 is a graph diagram illustrating an example of the
amount of line width distortion caused by varying the line pitches
for one embodiment of a maskless optical direct write lithography
system constructed in accordance with the principles of the
invention.
[0021] FIG. 3 is a simplified schematic diagram illustrating one
embodiment of a mirror array suitable for use in a lithography
system accordance with the principles of the invention.
[0022] FIGS. 4(a)-4(b), 5(a)-5(b), and 6(a)-6(b) are simplified
plan views of portions of mirror arrays that schematically depict
array configurations used to produce high quality phase shift
patterns in a maskless optical direct write lithography process in
accordance with the principles of the invention.
[0023] FIG. 7 is a graph diagram illustrating an example of the
amount of phase adjustment used in assist features for various line
pitches for one embodiment of a maskless optical direct write
lithography system constructed in accordance with the principles of
the invention.
[0024] FIG. 8(a)-8(g) are schematic depictions of various mirror
embodiments and configurations implemented in a maskless optical
direct write phase shift optical lithography system constructed in
accordance with the principles of the invention.
[0025] FIG. 9 is a flow diagram illustrating operations in
performing a maskless optical direct write phase shift optical
lithography process to pattern a substrate in accordance with an
embodiment of the present invention.
[0026] It is to be understood that in the drawings like reference
numerals designate like structural elements. Also, it is understood
that the depictions in the Figures are not necessarily to
scale.
DETAILED DESCRIPTION
[0027] The present invention has been particularly shown and
described with respect to certain embodiments and specific features
thereof. The embodiments set forth hereinbelow are to be taken as
illustrative rather than limiting. It should be readily apparent to
those of ordinary skill in the art that various changes and
modifications in form and detail may be made without departing from
the spirit and scope of the invention.
[0028] In the following detailed description, methods and apparatus
for implementing optical direct write lithography systems and
processes are set forth. Such systems can employ maskless phase
shift lithography processes using off-axis light to establish
optical image patterns.
[0029] Maskless phase shift lithography as presently disclosed is a
valuable patterning technique due to the ready reconfigurability of
a programmable mirror array used to assist in pattern generation.
Previously, maskless optical direct technologies using phase shift
patterns required two or more separate exposures of the target
substrate to effectively transfer a pattern to the target
substrate. Some embodiments of the present invention can
advantageously be used to transfer patterns to the target substrate
using only a single exposure. Accordingly, throughput using such
systems can be substantially increased, and in some embodiments
throughput can be at least doubled.
[0030] The embodiments of the present invention utilize an off-axis
light beam directed onto the mirror array of a maskless optical
direct write system to generate a phase shift optical pattern which
is directed onto a target substrate to achieve pattern
transfer.
[0031] Embodiments of the invention are constructed to direct
off-axis light onto a mirror array to produce phase shift optical
patterns. The inventors have discovered that improved process
windows can be achieved for phase shift optical lithography by
incorporating this principle into a maskless phase shift optical
direct write lithography system. For example, embodiments of the
invention can be constructed to generate off-axis light beam(s)
that are directed onto a mirror array that is configured to
generate phase shift optical patterns. Off axis light beams are
light beams directed onto the mirror array at an angle other than
normal to the plane of the mirror array. Such configurations result
in photolithographic processes having improved process windows.
Such off axis illumination can be used to improve the process
windows ordinarily obtained using normally illuminated alternating
phase shift optical lithography (alt-PSM). By directing light onto
the mirror array at a non-normal angle, substantially all zero
order light is removed from the resulting optical signal. This is
an extremely attractive attribute for a phase shift optical
system.
[0032] Moreover, as the drive toward smaller and smaller critical
dimensions (CD's) continues, phase shift optical lithography
becomes even more attractive as a process technique. As feature
sizes and CD's decrease, light diffracted by such features scatters
at greater and greater scattering angles. Thus, systems having
their process windows optimized for small CD's encounter certain
difficulties when applied to features on the same surface having
lower pitch densities or greater line pitch distances. The
inventors have discovered that by offsetting the angle of incidence
(directing the light onto the array at an angle other than normal)
for an incident light beam, much improved (wider) process windows
can be achieved.
[0033] Removing zero order light is attractive because such light
is not generally useful for obtaining phase information. However,
in accordance with some embodiments of the invention, by using
off-axis light, the zero-order light (which does not include phase
shift information) can be substantially reduced in the resultant
reflected signal. The resulting phase shift optical signal (which
is projected down onto a target surface) has a substantially higher
portion of its signal having phase information. However, using such
off axis light results in some difficulties. For example, when a
line width of a certain line is patterned using off axis light,
line widths can vary as the pitch (number of lines per a given unit
length) varies, leading to distortions in the resulting line width.
These distortions from the desired line widths are referred to
generally as pattern drift or line width drift. This problem is
particularly troublesome when optical settings optimized for tight
(small or narrow) CD's are used to image looser pitches and larger
feature sizes on the same layer. The inventors have discovered that
by adding assist features in addition to off axis light, small CD's
and short pitch features can be imaged along with looser pitches
and larger feature sizes.
[0034] FIG. 1(a) is a simplified schematic diagram illustrating one
embodiment of an optical direct write system configured in
accordance with an embodiment of the present invention. The system
100 uses an illumination source 108 to generate an off axis light
109 that is projected onto a mirror array 102. In one
implementation the source 108 includes an aperture 108a. Thus,
light generated by the source 108 passes through the aperture 108a
which generates off-axis light beams 109 that are directed into a
beamsplitter 110 and through an optical system 112. After passing
through optical system 112, the off-axis light beams 109 are
directed onto a mirror array 102. The mirror array 102 is
configured to generate controlled phase differences in the light
from the off-axis light beams 109. Light reflected by the mirror
array 102 is passed through the optical system 112 and
beamsplitters 110, 114 and directed onto target wafer 104.
[0035] The illumination source 108 may be any illumination source
capable of generating electromagnetic waves sufficient to reflect
from the mirror array 102 and to induce chemical changes in a
photosensitive layer on the wafer 104. Preferably the illumination
source 108 is an intermittent source, capable of exposing the wafer
during selected periods of a continuous scan movement of the light
beam relative to the wafer. Commonly (but not exclusively), the
illumination source 108 is a coherent light source. In one
embodiment the illumination source 108 is an ArF excimer laser
producing 193 nm (nanometer) output. The optical system 112 is
typically a demagnifying projection optical system of a type known
to those having ordinary skill in the art. However, many types of
optical systems can be implemented. Moreover, the inventors
contemplate systems without such optics.
[0036] The off axis light 109 from the source 108 is directed onto
a mirror array 102 and projected onto the target substrate (here
target wafer 104) using, for example, beamsplitters 110, 114 and
projection optical system 112. As is known to those of ordinary
skill many possible arrangements can facilitate projecting a
desired light pattern onto the substrate in accordance with the
principles of the invention. In particular, in one alternative
configuration the projection optical system 112 can be arranged
beween the beamsplitting optic 114 and the wafer 104. Additionally,
it is especially pointed out that arrangements having fewer or no
beamsplitters can be used.
[0037] FIG. 1(b) is a simplified schematic cross section depiction
of a portion of an apparatus embodiment depiction of capable of
generating off-axis light beams that are directed onto a mirror
array in accordance with the principles of the present invention.
In particular, a source 120 embodiment is depicted. The
beamsplitters (e.g., 110, 114) and demagnification and focusing
optics (e.g., 112) have been dispensed with in this depiction to
simplify the explanation. A source 120 and included aperture 121
are shown. The source and aperture can be implemented, for example,
as 108, 108a of FIG. 1(a). Additionally, a mirror array 126 is
configured to generate a phase shift optical pattern. The array can
be the programmable array 102 (of FIG. 1(a)) and is positioned so
that light from the source diffracts from the array 126. The
depicted source/aperture combination are capable of generating
off-axis light beams that are directed onto a mirror array in
accordance with the principles of the present invention. As
previously explained, the source 120 can be implemented as any one
of a number of different light sources known to those of ordinary
skill. Particular embodiments can include coherent sources like
lasers or nearly coherent devices like filtered light sources. In
one embodiment, the source 120 can be a 193 nm ArF laser.
[0038] The depicted embodiment of the source aperture 121 is
configured to generate off axis light beams 122 in accordance with
the principles of the invention. Generally such off axis light
beams 122 are symmetrically arranged about the axis 124. For
purposes of this patent, the axis 124 comprises a line normal to
the plane of the mirror array 126. Thus, off axis light beams 122
are directed onto the mirror array 124 at an angle other than
normal to the array surface plane. Thus, off axis light is chosen
because it accentuates the contribution made by non-zero order
light. In fact, when light is directed onto a mirror array
configured as a phase shift optical pattern, substantially all of
the zero order light is removed from the diffracted signal. This is
advantageous because such zero order signals contain no phase
dependent information. Thus, the resultant diffracted signal 128 is
comprised almost entirely of first order light and higher order
light which is rich in interference patterns generated by the phase
shift mirror array. In general, optical systems having a .sigma. in
the range of about 0.50 to 0.90 are particularly well suited to
this invention. Additionally, the inventors point out that many
other .sigma. values can be used to generate suitable off axis
light beams. .sigma. values can be optimized to accommodate the
pitch of the wafer pattern in accordance with known principles such
as illustrated in the following equation:
NA*.sigma.=.lambda./2p
[0039] where NA reference to the numerical aperture of the optical
system; where p=wafer pitch; and where .sigma.=an aperture value
for an aperture of the optical system (e.g., a center aperture);
and where .lambda.=the wavelength of the exposing light beam.
[0040] For example, if the pattern to be formed on the wafer has a
pattern pitch of 180 nm (i.e., the pitch of an alternating pattern
of light and dark regions, for example, a series of 60 nm wide dark
lines separated by 120 nm wide "bright" spaces or alternatively a
series of 90 nm dark lines separated by 90 nm "bright" spaces, and
so on), the above equation can be used to determine the necessary
.sigma. values. In one example, using a pitch of p=180 nm (e.g., a
60 nm line with a 120 nm space), NA=0.70, and a .lambda.=193 nm
then a suitable .sigma. value is determined by
(193/(2*180*0.70)=0.77. This can be used as the center .sigma.
value. In order to obtain sufficient optical strength in the light
beam a sufficient gap must be formed between inner and outer
.sigma. values. In one example, an inner annular .sigma.=0.62 can
be selected along with an outer .sigma. value=0.92. In general,
inner and outer .sigma. values can range from about 0.5 to about
0.9, but as is known to those having ordinary skill in the art,
other values can be implemented.
[0041] FIG. 1(c) illustrates a face on view of one aperture
embodiment suitable for implementing the principles of the present
invention. The aperture 130 is configured to generate an annular
off axis beam pattern suitable for directing onto a mirror array
configured to implement a phase shift optical pattern. The annular
aperture 130 is configured having a blocking center portion 131
that defines the inner radius R.sub.i for the annular off axis beam
pattern. Also, the annular aperture 130 is configured having an
outer blocking portion 132 that defines the outer radius R.sub.o
for the annular off axis beam pattern. Such an annular aperture is
well suited to providing a set of equally distributed off axis
light beams 122 in accordance with the principles of the present
invention. In one particular embodiment, using for example a 193 nm
source and a 0.70 NA optical system, an inner radius having a
.sigma. value of about 0.51 and an outer radius having a .sigma. of
about 0.85 are satisfactory. In the depicted embodiment, the outer
radius of the outer blocking portion 132 has a .sigma. value of 1.
As is known to those of ordinary skill, other a values can be used
to optimize other systems.
[0042] Additionally, the inventors contemplate various other
aperture configurations that can produce satisfactory results in
accordance with the principles of the invention. For example, a
quadrapole aperture can be used. FIG. 1(d) depicts one embodiment
of such a quadrapole aperture 140 embodiment. Four openings 141 are
used to pass off axis light through the aperture 140. In the
depicted embodiment the openings 141 are symmetric in
configuration. A related approach using a "quasar" aperture can
also be used. In one common implementation known to those having
ordinary skill in the art a quadrapole quasar aperture is an
annular aperture broken up onto four separate segments.
[0043] In another embodiment, depicted in FIG. 1(e), an octopole
implementation can be used. Eight openings 151 are used to pass off
axis light through the aperture 150. Again, the openings 151 can be
symmetrically arranged about the aperture. The inventors note that
any number of openings could be used to generate the desired off
axis light beams. Even a single off axis aperture (i.e., a single
aperture displace from a centerline) or a dipole aperture could be
used. Also, the inventors point out that the openings need not be
circular. Other shapes and sizes can be used to practice the
invention. Additionally, the apertures can be configured so that
the light roll off pattern is abrupt (e.g., normal) or
alternatively a more gradual intensity increase can be employed
(e.g., a Gaussian intensity roll off). One of the difficulties with
such off axis illumination when applied to "large" open areas
(i.e., areas having pitch distances of greater than on the order of
the wavelength of the exposing light) is the previously mentioned
drift problem. Also, periodic optical patterns of diffracted light
also can extend into regions that are intended to be dark. These
difficulties can be corrected by configuring the mirror array to
generate an optical pattern that corrects for said line width
drift. This is done by configuring the array to produce "assist
features" that are formed using the mirrors of the mirror array in
accordance with descriptions provided below. The assist features
are added to reduce the intensity of the unwanted bands of light.
For example, the periodic bands of light in the "dark regions" can
be reduced to an intensity level below the threshold of light
required to convert the photoimageable material on a substrate
surface. Such photoimageable materials are alternatively referred
to as photosensitive materials. One commonly used family of such
materials are photoresist materials. The threshold level of light
exposure required to convert such photoresist materials is well
known to those of ordinary skill. For purposes of this patent
"convert" means cause a light induced reaction in a photosensitive
material such that the material becomes easily removable in
accordance with standard photoresist developing techniques.
Alternatively, "convert" can mean cause a light induced reaction in
a photosensitive material such that the material undergoes a
reaction that makes the exposed photoresist difficult to remove.
Both such processes are well known in the photolithographic
arts.
[0044] FIG. 2 is a simplified schematic graph diagram illustrating
aspects of the line width drift problem. When a system is optimized
for a pitch of, for example, 130 nm (e.g., a line pitch of 130 nm),
projected images corresponding to a line width of 60 nm can be
easily maintained. However, as depicted, when the line pitch of the
pattern becomes wider, the line width for the "60 nm wide" line
begins to drift from the intended value. For example, FIG. 2
depicts line width (201) as a function of line pitch (202). As
indicated here, as the line pitch broadens the resulting line width
becomes considerably narrower. For example, for a system optimized
for line pitch at, for example, 130 nm, at line pitches below 180
nm there is, as expected, fairly accurate reproduction of the line
width at the desired 60 nm width. However, as the line pitch widens
the line width drifts. For example, at a pitch of about 320 nm, a
corresponding line width of 30 nm is produced by the pattern (see,
205). Heretofore, this phenomenon has lead to serious manufacturing
difficulties when dense patterns (e.g., narrow pitches) and less
dense patterns are formed on the same layer. Consequently, there is
a need for a method for correcting this drift. Accordingly, the
inventors have provided a means for correcting this drift. The
mirror array is configured to include assist features (also
referred to as corrective features) that reduce the effect of this
line width drift.
[0045] Also, due to the nature of off axis illumination, a periodic
"banding" pattern of light and dark regions will extend into the
regions between the dark lines (i.e., the regions that define the
pitch density). In order to address this problem, an optical
pattern that corrects for said line width drift is generated (e.g.,
assist features) to cause the patterns to appear "pseudo-dense" by
causing a series of light bands which preserve the diffraction
pattern but are of "sub-resolution intensity". As used herein,
sub-resolution intensity means that the bands of light are present,
but that the intensity of the bands is so low that they are beneath
the conversion threshold for the photoimageable materials onto
which they are projected. This means that although the light
pattern is projected onto a region between the lines (regions that
have intentionally bright bands of light), the intensity is so
reduced that it does not react the photosensitive material.
[0046] Thus, some embodiments of the invention make use of a mirror
array configured to generate a phase shift optical image pattern
having a background bright pattern (defining a bright region fully
exposed); a pattern of dark lines (defining unexposed regions); and
a pattern of "gray lines" generated by assist features and having
an intermediate exposure intensity below the threshold of the
photoimageable material. For example, the background pattern can be
configured to generate a relatively intense exposure pattern
sufficient to activate the photoimageable layer formed on the
target substrate. In contrast, the pattern of dark lines can be
generated by configuring an associated portion of the mirror array
to generate a relatively intense destructive interference pattern
generating a pattern of dark lines by not activating the associated
portions of the photoimageable layer. Additionally, a portion of
the mirror array is configured to generate a pattern of "gray
lines" that demonstrate an intermittent light pattern having a
light intensity insufficient to activate the photoimageable layer
formed on the target substrate. Thus, although a phase shift light
pattern is produced having dark features (i.e., the so-called gray
lines) in an area that is not desirable to convert, the faintness
of the gray line optical signal is such that the pattern does not
transfer to the photoimageable layer. Of course for alternative
photoresist types, the light intensities are reversed. Thus, a
mirror array configured with assist features enables an apparatus
having optical settings (e.g., focus and dose) optimized for the
smallest CD to be used to faithfully transfer image features having
much larger critical dimensions without the need for additional
exposures. Moreover, these assist features permit proximity
correction and correct the effects of line width drift. Details of
this aspect of the invention are explained in greater detail
herein.
[0047] FIG. 3 schematically illustrates one embodiment of a mirror
array 300 suitable for use as mirror array 102 of FIG. 1. The
depicted mirror array 300 is depicted as a piston mirror
implementation arranged in a neutral position, in accordance with
one embodiment of the present invention. That is, the mirror array
300 illustrates one configuration of individual mirrors 302. The
individual mirrors 302 are shown in a coplanar position with the
plane of the mirror array 300, thus reflecting incident light back
in a direction normal to the plane of the individual mirrors 302
(as well as the plane of the mirror array 300). In such a
configuration, the incident light is reflected and directed to the
corresponding pixel of the wafer 104. Accordingly, with the use of
a positive photoresist layer on the wafer 104, the exposed region
will be converted and dissolved away (on the photoresist layer) for
subsequent etching operations. With the use of a negative
photoresist layer on the wafer 104, the exposed region will be
converted and left in place (on the photoresist layer) for
subsequent etching operations. It is to be understood that this
neutral position may be achieved by supplying suitable
electrostatic potentials to electrodes corresponding to the
individual mirrors as known to those of skill in the relevant arts.
With the use of a positive photoresist, in order to form a line in
the wafer, the mirrors require an adjustment in position such that
an absence of light (a dark line) appears on the selected portion
of the photosensitive resist on the wafer, thus allowing the resist
in the selected area to subsequently remain behind after
development followed by etching using the patterned resist layer.
Generally, dark areas correspond to a light pattern having complete
destructive interference. As is readily apparent to those of
ordinary skill, alternative photoresists change the effects of
light and dark to generate a desired photoresist pattern after a
development process. In one embodiment, the mirrors are formed of
aluminum or silicon with a top layer formed of aluminum. However,
any suitably stiff and/or reflective materials can be employed.
[0048] The present invention, in various embodiments, may be
configured to direct light to a substrate such as a wafer by
tilting the mirrors or arranging the individual mirrors in
piston-displaced positions or by using a combination of tilting and
piston displacement of the mirror surfaces.
[0049] FIGS. 4-7 illustrate some example configurations for the
mirror array in order to introduce phase differences into the
direct write printing process and thus to effectuate printing of
features. While illustrative, these configurations are not intended
to limit the scope of the present invention.
[0050] As previously explained, embodiments of the invention have
been constructed that can expose substrates using a phase shift
pattern in a single exposure. In order to achieve such patterns,
the mirror array is configured to generate a background pattern (of
light or dark) and a line pattern (of dark or light lines (the
opposite of the background)) augmented by a pattern of assist
features ("gray lines") formed at various points in the pattern. In
an embodiment that defines line patterns by exposing the
photoresist to light, the pattern of assist features forms a
sub-resolution light pattern (gray lines) that has an intensity
below the exposure threshold of the photoimageable material used to
transfer a pattern to a substrate.
[0051] FIG. 4(a) is a plan view of a portion of a mirror array 400
configured to generate a strong phase shift pattern in accordance
with the principles of the invention. The embodiment depicted in
FIG. 4(a) schematically depicts a mirror array configuration
capable of generating strong phase shift optical pattern having a
line pitch 404 of 180 nm and a line width 402 of about 60 nm using
a 193 nm source. The mirrors are, for example, demagnified to
achieve pixelization of about 30 nm.times.30 nm when projected onto
a substrate surface. The depicted pattern can be extended upward
411 and downward 412 to construct long lines. Additionally, the
number of lines can be increased by extending the pattern to the
left and right repetitively until the desired line pattern is
formed. The mirror array 400 is configured to include a background
pattern of mirrors 401 arranged to have zero phase. These mirrors
are adjacent to another set of mirrors 403 which are configured to
generate a phase difference of 180 degrees relative to the
background pattern of (zero phase) mirrors 401. Thus, the mirrors
create destructive interferences at the interface between the zero
phase mirrors and the 180 degree phase mirrors 403 to generate a
dark line about 60 nm wide. Alternatively, because all that is
important is relative phase, the mirrors 401 can be set at 180
degrees phase and mirrors 403 can be set at zero phase.
[0052] FIG. 4(b) schematically depicts a side view of the piston
mirror implementation of the depicted embodiment. It is noted that
mirrors 403 were displaced downward 180.degree. (i.e.,
-180.degree.) relative to the zero phase mirrors 401 but could just
as easily been displaced upward 180.degree.. Also, other
configurations could readily be applied.
[0053] However, as the line pitch is increased the line width of
the features produced begin to change. This phenomenon is in part
dependent of exposure source wavelength and the diffractive effects
of other portions of the mask. Thus, different degrees of
correction are generally required for different exposure
wavelengths. That being stated, the following examples will be
discussed with respect to a 193 nm exposure source. The invention
is, however, not so limited.
[0054] For a 193 nm source, drift from an ideal phase shift optical
pattern begins to distort the fidelity of the phase shift pattern
at line pitches of greater than in the range of about 110 nm to
about 150 nm. This is dependent of course on a number of factors
including, but are not limited to, the NA of the system and the
.sigma. of the source aperture. This problem can be quite severe.
For example, in a system optimized for a CD of about 60 nm, if a
portion of the pattern has a line pitch of above 320 nm the
resultant line distortions will cause a 60 nm wide line to print as
a line having a width of about 25 nm. Correction of this problem is
needed.
[0055] In the example provided by FIGS. 5(a) and 5(b) a line pitch
505 of 240 nm is offered as an example. With reference to FIG. 2 it
can be seen that for a 240 nm line pitch, the line width drifts
from the intended 60 nm to about 50 nm. This can be corrected by
adjusting the mirror array to produce an assist feature. Here such
a feature is introduced by adjusting the phase of feature mirrors
506. So the mirror array 500 is configured to include a background
pattern of mirrors 501 at zero phase. These mirrors are adjacent to
another set of mirrors 503 which are configured to generate a phase
difference of 180 degrees relative to the background pattern of
mirrors 501. These mirrors 503 are used to define 60 nm wide lines.
Mirrors create destructive interferences at the interface between
the zero phase mirrors 501 and the 180 degree phase mirrors 503 to
generate a dark line about 60 nm wide. Alternatively, because all
that is important is relative phase, the mirrors 501 can be set at
180 degrees phase and mirrors 503 can be set at zero phase.
However, because the line pitch has been extended to 240 nm, line
width drift has been introduced that will make the line produced by
mirrors 501 too narrow. This is corrected by adjusting the phase of
alignment feature mirrors 506. Here these mirrors are set at about
57 degrees phase relative to, for example, the mirrors 503.
Additionally, small phase adjustments can be made to the mirrors
506 to further fine tune the position and width of lines produced
by the array 500. Generally, the alignment feature mirrors 506 are
set in the range of about 50-140 degrees relative to the adjacent
mirrors.
[0056] In another example, FIGS. 6(a) and 6(b) depict a portion of
a mirror configuration designed to correct aberrations in a line
pitch of 900 nm. Additional, identically configured, portions of
the mirror array can extend in repeating patterns to the right and
to the left of the depicted portion. Thus, in the depicted
embodiment, for a 900 nm line pitch, five assist features 606 can
be used for each line 603. As before line width drift correction is
implemented to return the intended line width to about 60 nm. Here,
several of the patterns are adjusted by adjusting the phase of
assist feature mirrors 606. As before, the mirror array 600 is
configured to include a background pattern of mirrors 601 at zero
phase (here accomplished by three mirror wide groups 601). In the
middle of the pattern is a set of mirrors 603 configured to
generate the line. This set of mirrors 603 is configured to
generate a phase difference of 180 degrees relative to the
background pattern of mirrors 601. Again the line defining mirrors
603 are set to define 60 nm wide lines. Again, as before, because
all that is important is relative phase, the mirrors 601 can be set
at 180 degrees phase and mirrors 603 can be set at zero phase.
However, because the line pitch has been extended to 900 nm, a
large amount of line width drift has been introduced that will make
the line produced by mirrors 601 too narrow. This is corrected by
introducing a correction using mirrors 606 (here five sets) to
generate assist features and adjusting the phase of these mirrors
606. Here, in the depicted embodiment, these mirrors 606 are set at
about 74 degrees phase relative to, for example, the mirrors 603.
As before, small phase adjustments can be made to the mirrors 606
to further fine tune the position and width of lines produced by
the array 600. Generally, the mirrors 606 that form the assist
features are set in the range of about 50-140 degrees relative to
the adjacent mirrors.
[0057] The degree of phase adjustment for the correction feature
generating mirrors is graphically depicted in FIG. 7. The depicted
relationship describes the amount of phase difference (for the
correction feature generating mirrors relative to the background
pattern mirrors (e.g., zero phase difference) required to maintain
a 60 nm line at 60 nm for each line pitch. For example, at line
pitch of 240 nm, the phase difference for the correction feature
generating mirrors is about 57 degrees (relative to the background
zero phase). Also, at line pitch of 300 nm, the phase difference
for the correction feature generating mirrors is about 62 degrees
(additionally, in some embodiments, the array is configured to
generate one assist feature per line), for a line pitch of 420 nm,
the phase difference for the correction feature generating mirrors
is about 78 degrees (additionally, in some embodiments, the array
is configured to generate two assist features per line), for a line
pitch of 540 nm, the phase difference for the correction feature
generating mirrors is about 80 degrees (additionally, in some
embodiments, the array is configured to generate two assist
features per line), at line pitch of 900 nm, the phase difference
for the correction feature generating mirrors is about 74 degrees
(additionally, in some embodiments, the array is configured to
generate five assist features per line), and so on.
[0058] The foregoing systems and methods use programmable optical
mirrors in a maskless lithography system to form desired optical
patterns on a substrate. Such systems can make use of a number of
different programmable mirror arrays. For example, the described
maskless direct-write lithography system can use mirrors configured
to operate in a piston-displacement ("piston") mode; a cantilevered
(i.e., hinged at one end) mode, a torsional (center twisting) mode
having a quarter wavelength plate on a portion of the mirror or
capable of operating in one or more modes at the same time. A
scanning apparatus can be used to expose various portions of a
substrate to a pattern produced by the optical portions of the
system (in those cases where an entire substrate is not exposed at
one time). By using the proposed inventive system, light from a
source may be modulated by combining the phases from the mirrors in
a customized fashion to form the desired pattern. The individual
mirrors are controlled to implement the above-referenced phase
shift patterning techniques.
[0059] The present invention uses an array of mirrors that
introduce phase differences into an optical signal which is
projected onto a target substrate to form an image pattern that is
used to establish patterning of a photosensitive layer formed on
the substrate. In some embodiments the mirrors are programmable
(i.e., the mirrors can be actuated using a controller) allowing the
system to individually (or collectively) orient mirrors to
introduce a light path difference (i.e., across the mirrors)
resulting in a phase difference that enhances contrast in a
resultant image. Moreover, such phase differences can be used to
generate light regions (which expose the target to relatively
bright light intensity calculated to expose a photoimageable layer
(i.e., a photoresist)) and dark regions (generally due to
destructive interference patterns) calculated to form dark lines by
not exposing the photoimageable layer. Also, in embodiments of the
present invention, intermittent patterns of lower intensity light
are used to enhance the process window of the system. These
so-called "gray-line" features and their use have been described
herein.
[0060] Referring back to the mirror array 102 illustrated in FIG.
1(a) preferably comprises a plurality of mirrors. The individual
mirrors preferably capable of a piston mode, a tilting mode (which
includes tilting about a center axis in a cantelivered mode or
tilting about a mirror edge in a hinged mode), or some combination
of tilt and piston displacement operation. Additionally, mirrors
operating in a "cantilevered" mode, a piston mode, an edge tilting
mode or some combination can include a mirror wherein a portion of
the mirror includes a quarter wavelength plate configured to be
highly transmissive at the wavelength of light used by the source
108. In the tilting, piston, and cantilevered mode, each individual
mirror is controlled by electrostatic voltages applied to
electrodes to control the degree of tilt or piston motion in an
analog fashion. The actual tilting or piston motion of the
individual mirror may be constrained to several degrees of movement
in either direction from the neutral "flat" position by the
physical configuration of the mirrors and the mirror array.
[0061] As explained above, a mirror array (e.g., 102, 126) is
implemented to practice the invention. Typically, each of the
individual mirrors in the mirror array is responsive to control
signals provided to orient the mirrors. Additionally, each of the
mirrors can be programmably actuated using, for example, a mirror
array control element. Referring, for example, to FIG. 1(a), such a
control element 115 can use software to actuate the individual
mirrors of the array 102 to produce a desired optical pattern which
is then projected onto a target substrate (here wafer 104) to
produce a desired image. As alluded to above, the off axis light
109 from the illumination source 108 may be directed to the
photosensitive wafer 104 by any suitable means as known to those of
skill in the relevant art. In accordance with one embodiment, the
mirror array 102 comprises a plurality of mirrors, each of the
plurality of mirrors having a very small size. For example, mirrors
having sides on the order of about 8 .mu.m (micron) can be used.
The inventors specifically point out that other sizes of mirrors
can be used. For example, Micronic Laser Systems produces a 16
.mu.m.times.16 .mu.m mirror array that could be used to implement
aspects of the invention. Such mirrors can be demagnified to any
final pixel size. In one embodiment, a final pixel size of about
27-30 nm on a side (at the image plane e.g., on the photosensitive
layer of the wafer 104) is employed. Such demagnification can be
accomplished using a number of lens elements 112 such as known to
those having ordinary skill in the art. Preferably, such
demagnification is accomplished so that each of the plurality of
mirrors corresponds to a pixel imaged on the wafer. Although the
apparatus illustrated is a catiotropic configuration, the scope of
the invention is not so limited. That is, any configuration which
allows the use of mirror arrays to direct light to a substrate is
expected to be suitable and thus within the scope of the
invention.
[0062] Reference to FIGS. 8(a)-8(e) schematically illustrate modes
of operation of each of the three mirror embodiment types. In each
of the depicted embodiments the top surfaces are reflective. FIG.
8(a) illustrates the mode of operation for a "cantilevered"
embodiment. The mirror 810 is tilted about an edge 811 (in the
direction indicated by the arrow) to displace the mirror a distance
such that light reflected by the displaced mirror has a different
phase that light reflected by other mirrors in the array (e.g.,
undeflected mirrors). FIGS. 8(b) and 8(d) illustrate the mode of
operation for a quarter wavelength tilt mirror embodiment. The
mirror 820 is rotated about a center post 823 as indicated. The
mirror 820 includes a quarter wavelength thick transmissive plate
821 formed over a portion of the mirror surface. As illustrated in
FIG. 8(d), the transmissive plate 821 is made so that it is about
one quarter of a wavelength 822 (of the exposing light) thick. This
means that if the source is a 193 nm wavelength laser, that
transmissive plate is at least partially transmissive at 193 nm and
about one quarter of the wavelength (193 nm) thick. The actual
thickness is chosen as one quarter of a wavelength because light
incident of the plate 821 penetrates about one quarter of a
wavelength into the plate 821 where it is reflected by the mirror
820 surface and travels another one quarter of a wavelength back to
the surface. At this point the light is 180 degrees out of phase
with the light reflected by the other (un-plated) portions of the
mirror 820 generating a destructive phase interference pattern.
Thus, as is known to those having ordinary skill in the art,
depending on the materials chosen and the light wavelengths used,
the quarter wavelength plate 821 is configured to generate
destructive interference relative to light incident on the mirror
820 and not passing through the plate 821. FIG. 8(d) schematically
depicts two adjacent mirrors being tilted in accordance with an
embodiment of the invention. The tilting is done to displace the
mirror edges a distance such that light reflected by the displaced
mirror has a different phase that light reflected by other mirrors
in the array (e.g., adjacent mirrors).
[0063] Finally, FIGS. 8(c) and 8(e) schematically illustrate a
piston type mirror embodiment. Such piston mirrors 830 can be
elevated or depressed such that the mirror faces remain
substantially parallel to each other regardless of the degree of
elevation or depression. By carefully selecting the degree of
elevation or depression of the mirrors, a mirror array can create
any number of interference patterns relative to adjacent mirrors
(or mirrors some distance away).
[0064] The movement of these mirrors is sufficient to alter the
phase of the light reflected by the mirrors (also referred to
herein as the mirror phase) such that adjacent mirrors can be
configured to provide controlled interference and accordingly vary
the amount of light reaching the photosensitive region of the
wafer. This results in enhanced contrast. It should be noted that
for illustrative purposes the configuration and use of the mirror
array are described in the context of a system applying a pattern
to a photosensitive region of the wafer. However, the invention
scope is not so limited. The scope of the invention is intended to
extend to transfer of patterns to any photosensitive layer, for
example to include the formation of patterns on photo masks or
reticles.
[0065] Additionally, the inventors teach that an alternating
pattern of mirror displacement arranged so that each mirror is 180
degrees out of phase with each adjacent mirror is effective for
creating destructive interferences that can be used to create large
dark regions. On example of such a pattern is disclosed in the plan
view of a portion of a mirror array embodiment 800 depicted in FIG.
8(f). The light mirrors 801 a configured to diffract light so that
it is 180 degrees out of phase with light from the dark mirrors
802.
[0066] Structures and operational use of tilted and mirror arrays
are known in the art and thus further detail here is deemed
unnecessary. For example, the use of piston and tilted mirrors is
described in "Optical Analysis of Mirror-Based Pattern Generation"
by Y. Shroff, Yijian Chen, and W. G. Oldham; Proceedings of SPIE,
Vol. 5037 (2003), the entire disclosure of which is incorporated
herein by reference for all purposes.
[0067] As a further example, integrated circuits comprising
microelectronic mirror devices are available commercially. For
example, Texas instruments, Inc. of Dallas, Tex. produces a Digital
Micromirror Device (DMD) comprising an array of microscopically
small square mirrors, each mirror corresponding to a pixel in the
projected image.
[0068] Additionally, piston mirror implementations are described in
the U.S. patent application Ser. No. 10/825,342, entitled:
"Optimized Mirror Design for Optical Direct Write", filed on Apr.
14, 2004 (Attorney Docket No. LSI1P239/03-180) previously
incorporated by reference for all purposes. And also described in
the U.S. patent application, entitled: "Process And Apparatus For
Generating A Strong Phase Shift Optical Pattern For Use In An
Optical Direct Write Lithography Process", filed on 18 Nov. 2004
previously incorporated by reference for all purposes.
[0069] Additionally, certain complications can arise when different
line widths are to be printed on the same substrate. In one
instance this arises when a system optimized for one line width has
other lines characterized by a different width formed on the same
layer. For example, if the optical settings are optimized for lines
60 nm wide with a pitch width of 180 nm, but a number of other
wider lines are also needed on the same layer. This can be
accommodated by making certain adjustments to the phase pattern
used to print the lines. For example, a series of "assist features"
can be introduced to into the pattern to enable the pattern to be
printed with the necessary fidelity. The problem is that the
principles of the invention depend on off-axis phase edge
interference where you have 100% transmission bright background at
(for example) 0 degrees, and 100% transmission features at (for
example) 180 degree phase. In other words, a field of mirrors is
configured to reflect light at 0 degrees and a field of mirrors is
configured to reflect light 180 degrees out of phase from the 0
degree mirrors. The regions where the 0 degree phased light and the
180 degree phased light touch each other produce phase interference
that results in the formation of a dark line. But as the width of
the 0 and 180 degree regions widen with increasing CD (e.g., as
when one moves perpendicular to the phase interference interface),
the interference effect of one phase on another decreases.
Consequently, the overall light intensity rises and accordingly the
CD cannot be maintained without lowering the dose. This has the
effect of fusing together the smaller CD lines. To make wider lines
using the same process conditions, two (or more) phase interference
features (hence more phase generating interfaces) are used.
Additionally, the features that generate the phase interfaces of
the patterns are separated by some distance to maximize the
effective width of the interference pattern. For example, if the
system is optimized for features 60 nm wide and lines 240 nm wide
are also desired, a second, third, and fourth, set of interfaces
can be provided. Each of the interfaces work together to generate a
resultant pattern that creates, for example, a broader "dark"
region. In one example, the sets of interfaces can be spaced at
intervals of about 60 nm apart to generate a pattern that appears
as if it were a single "line" of about 240 nm wide.
[0070] Additionally, where wide dark areas are desired checkerboard
phase interference patterns can be employed. See, for example, FIG.
8(f). Also, where small dark areas are used next to wide areas of
bright intensity (as might be used to print a series of widely
separated lines using a positive photoresist) the bright areas can
be populated by a number of periodically spaced assist features
that result in periodically spaced apart regions of lower light
intensity. As depicted in FIG. 8(g), these areas are configured to
reflect a lower light intensity, but not sufficiently low so as to
affect the solubility of the photoresist. This can be done by for
example by introducing a phase shift at the intermediate features
that alters the phase to some intermediate value other than 0 or
180 degrees. For example, the mirrors can be adjusted so that they
reflect light at a phase value in the range of about 45 degrees to
about 135 degrees. FIG. 8(g) provides a schematically depicted
light profile of one example implementation. The diagram 850 is a
schematic depiction of light dose (I) over distance (d). In the
depicted example a positive photoresist is used (although similar
principles can be applied to negative photoresists). The exposure
threshold 851 depicts the exposure level required to make the
resist soluble. Exposure levels below the threshold result in the
photoresist being insoluble. The depicted diagram illustrates an
exposure pattern used to generate a pattern of dark lines separate
by a large bright area in accordance with the principles of the
invention. Strong phase interference is generated in regions 852,
853 resulting in dark lines. To facilitate the formation of the
large "bright" region 854 between the dark lines, assist features
are used. A less pronounced phase interference is generated in
regions 855, 856 by the assist features. For example, a phase
difference of 90 degrees might be introduced. This will reduce the
light intensity in the regions associated with the resultant
interference pattern producing "gray lines". However, the reduction
in intensity is not so great as to cause the photoresist to remain
in place during development. Thus, the feature remains but the
light intensity is reduced to a sub-resolution level (i.e., the
gray lines do not form a pattern feature in the developed
photoresist layer). Thus, the phase pattern remains constant but
the relative light intensity is not constant resulting in a highly
controllable and predictable pattern having a high degree of
fidelity and image sharpness.
[0071] FIG. 9 is a flow chart illustrating operations in performing
direct write optical lithography using mirror arrays, in accordance
with one embodiment of the present invention. The process commences
in operation 902 with the configuration of the array. By
appropriately controlling the array and the stage, in a one-pass
operation, the selected features of the pattern may be created
using the configured mirrors of the array. In one implementation,
the mirrors are configured through the implementation of software
which individually (or in groups) configures the mirrors.
[0072] Next, in operation 904, the sample (e.g., wafer) is
illuminated using the configured array. In one embodiment a
continuous scan is performed with the exposure for each
configuration of the array occurring during a short interval of the
entire interval that the wafer is aligned with the corresponding
mirror array. Once the illumination is completed, the process ends
at operation 912.
[0073] Although the foregoing invention has been described in some
detail for purposes of clarity of understanding, it will be
apparent that certain changes and modifications may be practiced
within the scope of the appended claims. Accordingly, the present
embodiments are to be considered as illustrative and not
restrictive, and the invention is not to be limited to the details
given herein, but may be modified within the scope and equivalents
of the appended claims.
* * * * *